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Lecture Presentation

Fundamentals of Well Logging

Natural Gamma-Ray Logs and

their Interpretation

Carlos Torres-Verdn, Ph.D.

ProfessorDepartment of Petroleum and Geosystems Engineering

The University of Texas at Austin

Objectives:

1. To understand the physical principles behind theoperation of spontaneous gamma-ray logging tools,

2. To learn how to interpret gamma-ray logs in termsof clastic lithology, shale content, grain size, andsome other petrophysical properties,

3. To conceptually understand when and when notgamma-ray logs are indicative of shale/clayconcentration,

4. To introduce the concept of spectral gamma-raylogs, and

5. To understand what environmental corrections arecustomarily applied to gamma-ray logs.


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IMPORTANT REMARKS:

1. Clay/shale can substantially affect petrophysicalproperties of rocks such as porosity, irreduciblewater saturation, capil lary pressure, relativepermeabil ity, absolute permeabili ty, andpermeability anisotropy.

2. It is necessary to diagnose the specificdistribution of c lay/shale in the pore space, thetype of clay, and the volumetric concentration ofclay/shale in order to quantify the petrophysicalproperties of rock formations.

3. Presence of clay/shale affects practically ALL

well-log measurements.

4. Presence of clay/shale can cause electrical,permeabil ity, and elastic anisotropic behavior.

Examples of Turbidites:

Bouma Sequences


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Where are the Shales?


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Relationship between Grain Size Distribution,Pore Size Distribution, Throat Size Distribution,and Tortuosity: Influence on Permeability andCapillary Pressure

Turbidite Deposits / Submarine Fans


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Geological/Depositional Model

Dr. Galloways model

From Pirmez et al., 2000

Analogous Example: Of fshore

Nigeria, Niger Delta Slope

Geological/Depositional Model


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Shale, Silt, and Clay

FACTS:

1. Clays are naturally radioactive (theyspontaneously release gamma rays).

2. Most clays contain Th, U, and K. Clay/shaleconcentration increases with [Th, U, K]concentration.

3. In siliciclastic rocks, grain size often correlateswith presence of clay/shale.

4. Warning I: there are some rocks which have noclay/shale but do exhibit abnormalconcentrations of [Th, U, and/or K].

5. Warning II: Drilling mud can contain K.


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RADIOACTIVITY: THE BASICS

NATURAL RADIOACTIVITY OF ROCKS,NATURAL GAMMA RAY ACTIVITY


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GAMMA RADIATION

DEFINITION OF HALF LIFE


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QUESTION:

Why are shales/clays

naturally radioactive?

Natural Element Abundance in the Earths Crust

(After Darwin Ellis)


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What do clay and si lt (shale) have to do

with natural gamma ray activity?

What is a clay?Example of Clay-Coated Sand Grains


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What is a clay?

Example: Chlorite

Example: Pore-Filling KaoliniteWhat is a clay?


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Example: Pore-Bridging Illite

What is a clay?

Smectite

Chlorite

Kaolinite


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Illite

Glauconite



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Thick-bedded turbidite sand with discontinuous shale-clast horizons

Thick-bedded turbidite sands

Examples of Turbidites:

Bouma Sequences



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Where are the Shales?:The case of naturally radioactive sands


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W a r n i n g !Some Evaporites are Naturally Radioactive


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Sequence Boundary

Dolostone Bed

Cycle Top


Bentoniccla

ys

Limestone

Elongate

dchertnod

ules(silic

ates)



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Calibration ofGamma Ray

Detectors

SCINTILLATION COUNTERS


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250 cm 200 150 100 50 0 cm

80 cm

80 cm

40 cm

30 cm

20 cm

60 cm

5 cm2 cm

0 cm

INDUCTION LOG

LATEROLOG

NEUTRON

GAMMA RAY

DENSITY

SONIC

MICRO RESISTIVITYMICROLOG

DIPMETER

DEPTH OF INVESTIGATION

RESOLUTIO

NRESISTIVITY

RADIOACTIVITY

RESISTIVITY

ACOUSTIC

Logging Tools

TYPICAL GAMMA RAY RESPONSES


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SPECTRA FOR K, Th, and U

TYPES OF MEASUREMENTS


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INTEGRAL GAMMA-RAYMEASUREMENT

GAMMA-RAY API VALUES OF MINERALS


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GAMMA RAY LOG EXAMPLE

SPECTRAL GAMMA RAY LOG


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Bed-Boundary Effects

DEFINITION OF VOLUME OF SHALE

Rock = Liquids and Gases + Solids (Matrix)

Shale

Solid Component of the Rock

Volume of Shale =

ROCK

SOLIDS

LIQUIDS AND GASES

Total Rock Volume

Volume of Shale


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Clay and Sandstone

After Rabaute et al. (2003)

Core Vshale: Example


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EXAMPLE: Synthetic gamma-ray logs

Tide Influenced Delta

Frewins Castle Sandstone

Belle Frourche Member

Frontier Formation

Cretaceous (Cenomanian)

Tisdale Mountain Antic line, WyomingPhoto by Rob Wellner


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Definitions

Shale can be dispersed, laminated or

structural

Shale structure is not critical in

computing hydrocarbons-in-place. It is

important in determining producibility.

Shale structure can only be determined

from core or with image logs, like theFMI.

Laminated Shale

Shale laminae

occupy both pore

space and grain

space

e = ss - VshL ss

These laminae are at

the density resolution

limit. (sand grains not

to scale)

2feet


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Dispersed Shale

Dispersed shale

occupies only porespace

e = ss - VshD

ss or PHIMAX isthe maximum

clean sandstone

2mm

Structural Shale

Structural shale

occupies grain space

e = ss2mm


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EXAMPLE

0 10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

35

40

Csh (%)

Porosity(%)

LAMINATED

DISPERSED

STRUCTURAL

THOMAS-STIEBER PLOT


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Example: Thin Bedded Pay

Deep water facies thinbedded pays appear asorganized bands and havemoderate dips.

This facies makes goodreservoirs , even with avery high Vsh and lowinterval average PHIE.

They are hard to detectdue to low resistivity andhigh GR

Static Normalized Images

0 90 180 270

Static - Equal Increments - Linear64color

XScale = 1:4.2YScale = 1:10.0

ESTIMATION OF SHALE CONTENT


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Volume of Shale (Vsh)

Computation:

Empirical Technique

Shale Baseline

Clean Sand Baseline

min max

0

1

Ish

ESTIMATION OF SHALE CONTENT (I)


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ESTIMATION OF SHALE CONTENT (II)

M-10

M-20

M-40

M-50

M-60

M-30

M-

SeriesSands

cleansh

clean

GRGRGR

GRGRI

=

( )12*083.0 7.3 = GRIsh

V

where

EXAMPLE


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CLAY MINERAL IDENTIFICATION


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MINERAL IDENTIFICATION

Some Review Questions Part I):

1. Why is a rock naturally radioactive? What is thedefinition of half life of a radioactive substance?

2. What is a gamma ray? Why do gamma rays come indif ferent (quantum) energies (frequencies)?

3. What is a clay?

4. According to well-log practitioners, what is the definitionof shale?

5. Why are precisely the gamma-ray spectral signatures ofTh, U, and K used in well logging as indicators of shaleconcentration (amount of shale per unit volume)?

6. When will the gamma-ray spectral signatures of Th, U,and K not be indicative of shale concentration?

7. Are gamma-ray logs sensitive to the solid (matrix) or tothe fluid component of a rock, or to both?

8. Are gamma ray logs sensitive to mud-filtrate invasion?


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Some Review Questions Part II):

9. What is the radial length of investigation of naturalgamma ray tools? Does it matter if the rock is denserthan normal?

10.Why are spectral gamma-ray logs often used to assess

types of lithology?11.List at least five different geological situations that willentail the use of spectral gamma ray logs for theassessment of shale concentration.

12.What are typical values of gamma-ray readings in acarbonate sequence?

13.What are the typical values of gamma-ray readings in asiliciclastic sequence?

14.Why are tuffaceous sands naturally radioactive?

15.What are the environmental corrections that areapplied to gamma ray logs?

16.Unaccounted presence of barite in the mud, will itcause a sand to appear shalier or c leaner? Explainyour answer.

Baker Atlas

Schlumberger

Acknowledgements:

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